The present invention relates to a method and a device for removing from a liquid body a liquid mixture, which forms a surface layer on the liquid body, and separating this liquid mixture into one relatively light liquid and one relatively heavy liquid.

In workshops with machine tools for turning and cutting of metal pieces there is a need for a method and a device of the aforementioned kind for cleaning of liquids used for cooling purposes in the machine tools. Coolants of this kind normally are water based and deteriorate during use by, among other things, small amounts of oil being mixed with the coolant, such as hydraulic oil and lubricating oil, used for the operation of the machine tools. Oil of this kind causes an unpleasant smell and makes the coolant unusable in the course of time, if it is not separated from the coolant. It is known to clean coolants from oil by means of a centrifugal separator. In practice, this is performed such that a liquid mixture in the form of coolant and undesired oil is allowed to flow from a surface layer of a liquid body, containing the coolant to be cleaned, over an overflow outlet member into a collecting chamber. The liquid mixture is then pumped from the collecting chamber by means of a pumping device to the centrifugal separator.
Even in other connections there is a need for a method and a device for removing from a relatively heavy liquid, small amounts of a relatively light liquid. For example, liquids used for cleaning purposes often must have light floating contaminants removed therefrom, so that the liquid can be used anew.
The object of the present invention is to make possible cleaning of a relatively heavy liquid from small amounts of a relatively light liquid in a simple and non-expensive way by means of a simple and inexpensive device. The device should be compact and easily mountable to a container which contains liquid to be cleaned. Furthermore, the device should be operable automatically, and a desired cleaning effect should be obtainable without the need for complicated coordination of the operation of a centrifugal separator and a pumping device required for pumping the liquid to the centrifugal separator.
For achievement of this purpose the invention suggests a method of removing from a liquid body a liquid mixture, which forms a surface layer on the liquid body, and separating this liquid mixture into one relatively light liquid and one relatively heavy liquid, the liquid mixture being first caused to flow, as previously known, from the liquid body over an overflow outlet member into a collecting chamber and then pumped therefrom into a centrifugal separator. The method according to the invention is characterized in that the liquid mixture is pumped from the collecting chamber upwardly and into a rotating centrifugal rotor, forming part of the centrifugal separator, by means of a pumping member that is connected with the centrifugal rotor and is rotating therewith and extends down into the liquid mixture in the collecting chamber. Preferably, the overflow outlet member is vertically movable relative to the pumping member, further liquid mixture being caused to flow over from the liquid body to the collecting container in an amount per unit of time corresponding to the capacity of the pumping member and/or the corresponding centrifugal rotor.
For achieving a desired function of the vertically movable overflow outlet member a previously known technique may be used. Thus, the overflow outlet member may be kept floating on said liquid body, in accordance with U.S. Pat. No. 3,633,749, or be kept floating on the liquid mixture present in said collecting chamber, as set forth in U.S. Pat. No. 5,693,218. Alternatively, a different technology may be employed for automatic control of the amount of liquid mixture that is to flow over from the surface layer of the liquid body to the collecting chamber. A basic mission for the overflow outlet member is to adapt the flow of new liquid mixture into the collecting chamber to the flow of liquid mixture pumped up from the collecting chamber to the centrifugal rotor by means of the pumping device.
If the level of the surface layer of the liquid body is changed, it is desirable that the overflow outlet member automatically adapts itself thereto. This requires, if the collecting chamber is delimited by a collecting container having certain vertically immobile parts, that at least part of one wall of the collecting container is vertically movable together with the overflow outlet member.
In a preferred embodiment of the invention a collecting container wall part of this kind includes a bellows but, alternatively, sealing devices of different kinds may be used between the overflow outlet member and vertically immobile parts of the collecting container.
For cleaning of a liquid mixture, which has been transferred in the above described manner from the surface layer of a liquid body to a collection container for further transportation to a centrifugal separator, it is suggested according to the invention that the centrifugal separator includes a centrifugal rotor, which is connected with a pumping device adapted to extend from above and downwards into the liquid mixture present in the collecting container. The centrifugal separator further includes a driving device, which is adapted to drive the centrifugal rotor as well as the pumping device connected therewith wherein the pumping device is adapted to pump the liquid mixture to and into the centrifugal rotor. The pumping device may be designed in many different ways. Preferably, it includes a pumping member which is directly connected with the centrifugal rotor so that the common driving device may be adapted for rotation of both, the centrifugal rotor and the pumping member through one and the same transmission device. However, this is not absolutely necessary according to the invention. Alternatively, the driving device may be coupled separately to the pumping member, in which case a gear device of one kind or another may be used between the driving device and one of the centrifugal rotor and the pumping member. Most important is that an increased rotational speed of the pumping member, meaning an increased liquid flow to the centrifugal rotor, brings with it a corresponding increase of the separation capability of the centrifugal rotor as a consequence of an increased rotational speed thereof.
In a preferred embodiment of the invention a pumping device includes a tubular pumping member, which is rotatable around a substantially vertical rotational axis, the pumping member defining a central pumping channel for conducting liquid mixture from the collection container to the centrifugal rotor. Preferably, both the centrifugal rotor and the pumping member are rotatable around said vertical rotational axis, and in the preferred embodiment the pumping member, as mentioned, is directly connected with the centrifugal rotor for rotation together therewith. A pumping device of this kind does not subject the liquid to large shear forces. Shear forces of this kind are disadvantageous in this connection, since they cause undesired formation of emulsion of the two liquids to be separated from each other in the centrifugal separator.
The method and the device according to the invention may be used for continuous cleaning of a liquid, cleaned liquid being returned directly to said liquid body. The cleaned liquid is preferably returned to a level below the predetermined surface layer of the liquid body, so that the pre-separation having occurred in the liquid body is disturbed as little as possible. In the device according to the invention the returning is made by means of a stationary casing, which surrounds the centrifugal rotor and which has a returning member extending down into the liquid body to said level.
As mentioned, the rotatable pumping member extending down into the liquid to be cleaned is preferably tubular. For minimizing the wetted surface of the pumping member and/or for avoiding that liquid is pumped upwardly on the outside of the pumping member, the present invention provides a non-rotatable wall adapted to extend from above and down into the liquid mixture. During operation of the device, a sealing device provided in the collecting container is arranged to seal between the non-rotatable wall and the rotatable pumping member. The sealing device may have any suitable design. For instance, an annular lip gasket of rubber or some other elastic material may be supported by the non-rotatable wall and surround the pumping member and seal radially against the outside thereof. Alternatively, a similar annular lip gasket may be supported by the rotatable pumping member, so that by means of the centrifugal force it may be kept pressed radially outwardly against the surrounding non-rotatable wall.
In a preferred embodiment of the invention the sealing device includes an annular axially movable sealing member and means adapted to accomplish an axial sealing force between the non-rotatable wall or non-rotatable members connected therewith and the rotatable pumping member. The sealing member may be rotatable together with the pumping member, but preferably it is non-rotatable and adapted to be pressed axially against a sealing surface, preferably an end surface of the rotatable pumping member.
A non-rotatable wall of the aforementioned kind, which surrounds the pumping member, protects against unintentional contacts with the pumping member during rotation thereof.
If the centrifugal rotor is suspended from a flexible suspension device, said non-rotatable wall is preferably suspended from the same flexible suspension device for avoidance of relative pendulum movements between the rotatable and non-rotatable sealing surfaces of the sealing device during operation of the centrifugal rotor.